1. Field of the Invention
The present invention relates to an image forming method for forming an image on a photosensitive member and, more particularly, to an image forming method for forming a multicolor image on a multicolor image forming photosensitive member to be used for electrophotography.
2. Description of the Prior Art
For forming a multicolor image by the electrophotography, there have been proposed in the prior art a number of methods and apparatus therefor, which can generally be classified into the following categories. One method repeats the formation of a latent image using a photosensitive member in accordance with the number of colors to be separated and the development using a color toner to superpose the colors on the photosensitive member, or transfers the color to a transfer material upon each development thereby to superpose the colors on the transfer material. Another method uses an apparatus, which has a plurality of photosensitive members according to the number of colors to be separated, to expose optical images in individual colors simultaneously on the respective photosensitive members, to develop latent images formed on the respective photosensitive members with color toners, and to transfer the developed images sequentially to a transfer material thereby to form a multicolor image having superposed colors.
However, the aforementioned first method has to repeat the latent image formation and the development several times and is defective in that it requires a long time for recording the image and is difficult to speed up. On the other hand, the aforementioned second method uses a plurality of photosensitive members in parallel and is advantageous in high speed. However, the second method requires a plurality of photosensitive members, optical members, optical systems and developing means and has its apparatus complicated, large-sized and highly costing so that it is short of practicability. Moreover, both of the two methods have found it difficult to register the images, when the image formations and transfers are repeated several times, and are seriously defective in that they cannot completely prevent the color drifts of the image.
In order to solve such problems, I have previously proposed methods of recording a multicolor image on a single photosensitive member by a single image exposure. One of the methods will be described in the following.
Specifically, this method uses a photosensitive member which is prepared by arranging a photosensitive layer, which is photosensitive over the entire visible range, with an insulating layer which has a plurality of color separating filters (i.e., filters which have their individual filter portions substantially transmissive only to rays of predetermined wavelength ranges) combined in fine linear or mosaic shapes. First of all, the photosensitive member thus prepared has its whole surface exposed to an image to distribute charges according to separate image densities in a photoconductive layers underlying the respective filters (to form a primary latent image, as will be so called in the following). Then, a whole-surface exposure is conducted with light capable of being transmitted through a first color separating filter to develop only the photoconductive layer underlying said filter with a color toner in a color corresponding to the kind of the filter formed with an electrostatic latent image (which will be called a "secondary latent image") according to the intensity of the primary latent image formation, preferably, in a color complementary to the color capable of transmitting through the filter, followed by a uniform charging treatment. Then, for the individual color separate images, similar whole-surface exposures, developments and recharging treatments are repeated to form the multicolor image on the photosensitive member so that the multicolor image is recorded all at once on the transfer material by a single transferring treatment.
As to the filter, however, the method described above has practical difficulties in the preparation of a filter which ideally has transmissivity only in a specified wavelength range. In order to prevent the sensitivity of the photosensitive member from dropping, however, the spectroscopic transmissivity of the filter is desired to be as high as possible. In practice, a filter would its spectral transmissivity enhanced also has increased its transmissivity to other wavelengths.
On the other hand, the aforementioned method releases the charges from the photosensitive layer corresponding to a specified filter by exposing the whole-surface of the photosensitive member to specified light. As to the exposing light, however, the whole-surface exposing light generally has a wavelength distribution, and each filter has a little transmissivity to wavelengths other than its specified one. As a result, the whole-surface exposing light will release considerable charges from other filter portions. This means that potential patterns are formed in those other filter portions. Although sufficient exposure may be applied for the image formations in the NP and KIP methods using photosensitive members having transparent insulating layers according to the prior art, the present method cannot apply specified whole-surface exposure infinitely.